A split soda straw, some JB quick and some car wax on the screw threads will do the trick.
Fit the straw over the broken post, fill everything with JB quick, put in the screw with the threads waxed, and let the epoxy harden. When it is cured, remove the screw and clean up any excess epoxy.
When you put the screw back in, turn it backwards until it clicks, to make sure that you are using the original threads instead of cutting new ones in the epoxy.
